1. Field of the Invention
The present invention relates generally to an automatic clothes dryer, and more particularly, to an electric control circuit for use during an automatic drying cycle which provides thermal bias to an operating thermostat and timer run-out to advance a timer when the dryer is stopped in mid-cycle.
2. Description of the Prior Art
Clothes dryers have employed small heater elements disposed adjacent to an operating thermostat which senses exhaust air temperature from the dryer to provide thermal bias during a drying cycle. The provision of thermal bias causes the thermostat to operate at a lower exhaust air temperature and is commonly used to lower the exhaust air temperature at which the operating thermostat resets thereby reducing cycling of a main heater.
Voltage divider resistors have also been provided in dryer control circuits in series with a timer to drop the voltage applied to the timer. In at least one control circuit, a resistor is disposed in series with a 110 volt AC timer motor across a 220 volt AC line functioning as a voltage divider to prevent burn out of the timer.
A variety of control circuit configurations providing timer run-out are known in the art which enable timer motors to run through their cycles in the event that the drying cycles are interrupted, or upon some other unusual occurrence. Dryer control circuits in which a resistor provides thermal bias to a thermostat are known and are used in dryers currently manufactured by Whirlpool Corporation, the assignee of the present application. The provision of thermal bias is also shown in U.S. Pat. Nos. 3,401,464, where a bias heater is connectable between two power leads; 3,409,994 in which bias heat is provided once a 125.degree. thermostat is tripped; 3,112,187 and 3,318,016, which provide adjustable bias heat; and 3,031,768, which provides continuous bias heat.
Dryer control circuits disclosing the use of a voltage divider in series with a timer include: Whirlpool Corporation wiring diagram No. 695406 which illustrates the control circuit used on clothes dryers it currently manufactures, U.S. Pat. Nos. 3,942,265 and 4,132,008.
A dryer control circuit having the feature of timer run-out is disclosed in U.S. Pat. No. 3,806,308 wherein a timer is independent of a gas heater so that the timer is advanced to "off" should ignition of the heater fail to occur. In U.S. Pat. No. 3,159,465 a thermostat trip near the end of the dry cycle ensures timer run-out, while U.S. Pat. No. 3,196,553 discloses a safety thermostat to advance the timer for small loads. Circuits which provide timer run-out for a door open condition are disclosed in U.S. Pat. Nos. 3,394,466 and 4,083,118. U.S. Pat. Nos. 3,180,038 and 3,609,873 disclose means for removing power from a heater upon the occurrence of a fault condition, such as a power failure or overheated condition.
The prior art thus recognizes the desirability of providing bias heat to an operating thermostat, of providing voltage dividing to a timer motor, and of providing timer run-out in the event a drying cycle is interrupted.